If you read discussions critiquing retail World of Warcraft, especially in relation to vanilla WoW, I find that the dialog is mostly around the following concepts:
I think the player base is quick to blame accessibility, casualization, and blizzard's bottom line for the above problems, but I think the ideas are much more nuanced and often interlinked. In-depth:
Back in 2005, if you wanted to run Upper Blackrock Spire as the alliance, you probably spammed Trade Chat, General Chat, and (maybe) LookingForGroup for with a message like "LFM 1 tank 2 DPS UBRS" until folks joined. If you were smart you did this in a macro. When you wanted to run a different dungeon, you edited your macro to the other dungeon abbreviation. If you were trying to find a dungeon group to join, you would watch trade chat for this sort of spam.
If you saw a group you wanted to join, you would click the person's name and write "inv". Maybe they'd just invite you, sometimes they'd check your level/class by shift+clicking your name in chat. Sometimes they'd be looking for particular classes to avoid loot conflicts.
I often see it argued that this sort of spam advertising is valuable communication. I think that's nonsense. I think the game would be much better off by putting a UI on top of this interaction and keeping the chat channels clear of spam. As a group leader, let me list my group and see which players have listed themselves as wanting to do UBRS so I can shoot them messages/invites. As a player, let me list myself so I can see which group leaders are currently putting groups together so I can message them. Let me inspect their class/level/gear from the interface. Do not make it cross-realm.
Finally, for some economic spice, add a "bid" column. It's an economic fact that different classes will have different demands than others. Classic WoW had a massive tank shortage, for instance. Let players put in a bid of gold to be invited to the group. If you put in a negative gold amount, the rest of the party pays you for the run, and if you put in a positive gold amount, you pay the rest of the party for the run. This lets less desirable builds compensate for their off-meta choices with bribes, and lets groups attract tanks and healers without inflating the economy.
Flying for the first time was an amazing feeling, probably one of the coolest, most exciting experiences I've had in WoW. The anticipation as I was leveling up through the Burning Crusade after the all-nighter to hit max so I could fly was immense. Within a few months though, the player base realized the Faustian bargain they had made: in exchange for the power of flight, they had given up the feeling of danger. Previously engaging journeys through potentially dangerous lands, either because you were on a PvP server and could be ganked, or because you were running through elite mobs and could be dazed to death were now trivial.
Overnight, the tension evaporated. The world of WoW shrunk as the max mount speed increased from 2x run speed on my mechanostrider to 4.1x run speed on my gladiator flying mount. Folks didn't have to ride through valleys or remember how to navigate mountain terrain or follow roads, you just look at map markers and fly directly there, and were probably completely okay to alt-tab and look at something else. You're safe, after all.
Following this, the (reasonable) question became: "Why are we traveling at all?" If travel is riskless, uninteresting, and quick... why do it? I don't have a good answer there. A game like Ashes of Creation wants travel to be meaningful: there's always a chance that another player kills you on the road and takes your materials and certificates. That further plays into the bounty/crafting system. WoW, especially on PvE (originally labeled "Normal") servers don't have any of that.
It's no surprise then, that when group-finder eventually came, the group was put directly into the dungeon, as the journey had been rendered toothless.
Before I can talk about class design, I have to talk about what difficulty is. The linked article goes further in-depth, but the core idea is that every video game can be broken down into two components: "the puzzle" and "the beatmap". "The puzzle" is the part of the game where you're trying to translate the information on your screen and in your headphones into "the beatmap". "The beatmap" is which buttons and mouse motions you need to make and when. A game like guitar hero has a trivially easy "puzzle" - they give you a beatmap on screen. A game like chess has a trivially easy beatmap - once you've solved the puzzle you just click the piece you want to move and plop it down in the square you want to move it to.
Classic WoW started with incredibly simple puzzles and beatmaps for basically everyone but healers and fury warriors. Healers because you can always bring fewer healers on a speedrun if they play better (coordinating healing, consumable management, drink-walking, down-casting, mana management), and fury warriors because I still haven't seen a warrior parse that manages to spend their rage 98% efficiently across a speed clear. They eventually whirlwind when they should have bloodthirsted, or bloodthirsted when they should have waited to execute. High skill cap puzzle.
The beatmap for most vanilla classes is absurdly boring. Imagine if we put a guitar hero song to molten core frost mage gameplay, where frostbolt is the green button on the fret bar. Green, green, green, green, green every 2.5 seconds.
A lot of the problems are because of mana. Consider this: when your class is mana-based, you have a slowly dwindling, slowly regenerating resource that gives you all of your offensive options, all of your defensive options, and all of your utility options until you run out of mana, and then you have none of those. This means the only real decision you make is before the fight - "how do I play for short/medium/long fights" and "do I think this is a short/medium/long fight".
The play pattern in a solo or group setting becomes to wait for the casters to have full mana, go wild until they're tapped out, and then repeat.
Contrast this with the rage-based warrior (the original builder/spender). At the beginning of a fight, you don't have rage, and so your options are limited, and you need to ramp up to damage (which is a telegraph in PvP and is a good thing but that's a separate topic). Once you get rage, you have decision pressure between spending that rage on offense/defense/utility, whereas the mana user never has that pressure until they're manaless. Additionally, within those categories, you also have the further option to wait for more rage to use more rage-efficient options. For example, you might have heroic strike which only costs 15 rage, but does far less damage per rage than bloodthrist at 30 rage.
But, if you wait for that 30 rage, you might miss your damage window, or get stunned, or the enemy might use a defensive or move, etc. Damage now vs damage later.
The actual play result in Vanilla is that some classes were wildly better than others at a design level to the point where the players playing those classes were passed up in difficult content. Druids and paladins were forced into healing roles, even though they imagined themselves as shapeshifters or holy warriors respectively.
Blizzard shifted their design priority to a philosophy called Bring the Player. The idea was that Vanilla had several niches - mages had arcane intellect, priests had power word: fortitude, etc. When putting a group together, players had an optimal group composition, and if the class/spec didn't find that group they were reluctant to take you. Retribution paladins didn't find that composition. "Bring the player" attempted to bridge the gap by making sure that roughly any 5 man group of tank/healer/3 dps would be viable, and any 20 man group of 2 tank/4 healer/14 dps (roughly) would be viable. Bringing 14 rogues would still be dicey, but now you're not looking for a super-specific comp.
They accomplished this by bringing the classes more in line with each other. If previously only mages provided a crit buff, now maybe balance druids and rogues could do it too. If only rogues had temporary magic immunity, now so did death knights and retribution paladins.
At the same time, they also reworked the classes to give them significantly better puzzles and marginally better beatmaps. Previously a class might be pressing 3-4 buttons in a deterministic rotation. Now, they might be handling random buffs that affect their rotations, 2-4 different buttons that have high lowest-common-denominator cooldowns that all need to be pressed when available. (Exercise for the reader: work out how many game-states pressing an ability with a 7-second cooldown and another ability with a 10-second cooldown produces with a 1.5 global. off-off, 5.5-off, 4-8.5, off-4.5, 5.5-3, 2.5-off, etc). They gave the classes unique resources (sometimes multiple) to manage - you'd be building one resource while spending another (runes/runic power, combo points/energy).
The result for each class is that figuring out what the next button you should press often ended up looking like:
"Do I have a proc? How much resource1/2 do I have? Is the enemy in execute range? Are there multiple targets? Is my short/medium/long cooldown available? Will I get full value out of it? Which of my priority abilities are available? Does the mob have a special vulnerability/resistance window coming up soon?"
Then, once you've solved the puzzle, the beatmap looks like "mash 4 while solving the next puzzle".
In giving all the classes enough puzzle-complexity to keep 15-year-vets mentally engaged, they've also given all of the classes enough tools that the players start to say "hey wait a second! these classes are all the same with different animations!"
At a certain level, they always were, right? We were always just pressing keys on our keyboard and interacting with health bars and the main difference was the animations. Did it matter that the warrior's sword swung every 2.5 seconds while the mage's frostbolt was cast every 2.5? There's still a difference in terms of the toolkit, though less. I would surely take modern class design over Vanilla. I would love to see reduced puzzle complexity in exchange for increased beatmap complexity. Make me input multiple buttons to perform one action, like fireball inputs in fighting games.
This came, I believe, with the introduction of cross-realm battlegrounds, group-finder, and sharding technology. The game became more single-player and more accessible. Blizzard became able to handle launch day problems, as well as tell evolving stories in the same geographic area by putting players on different shards. Cross-realm battlegrounds and group-finder solved queue problems for the "I just have 30 minutes" player.
The cost was the sense of community. It might sound romantic, but in Vanilla I knew tons of folks on my server and they knew me. A big part of that was being forced to form groups and guilds with the same folks on your server over and over, and being forced to play against the same horde premades in Warsong Gulch every night. Solve queue problems by letting the factions play against each other (they always could use the practice after all). Offer low-population servers one-way transfers to medium-population servers. Point new players to medium population servers. Otherwise, make it difficult to transfer. A server should be a home and feel like one. If you want to go somewhere else, you should have to start over, not pay $25.
Solve group-finder problems using my above group-finder UI suggestion. Moderate chat channels to get the spam and boost advertising out so that players actually use it, and know to use it. Make chat for actual chat.
What happens to the players who truly only have 30 minutes to play and don't have time to meet people, get to a dungeon, run it, and chat? The players that only have time to teleport to a dungeon, speed-clear it, and then disappear back offline? Making the game support that kind of play is what destroyed the sense of community, so pick your poison.